The present invention relates generally to inductive windings for electrical apparatus such as transformers, reactors and the like and more particularly to spirally wound inductive windings of the continuous disc type having an electrostatic shield arrangement that significantly improves the transient response of such windings.
It is well known that highly inductive windings, as in iron core transformers and reactors, when exposed to steep wave front impulse or transient voltages exhibit initially, an exponential distribution of voltage drop along the length of the winding with a very high-voltage gradient along the first few turns or disc coils of said winding. This extremely non-uniform distribution of voltage is due primarily to the unavoidable distributed capacitance between each incremental part of the winding and adjacent grounded parts such as the core and casing structure of the transformer. Such ground capacitance is referred to as "parallel" capacitance. Such a winding also possesses a distributed capacitance between turns and groups of turns, the sum of such capacitance being in series with the winding terminals. If this "series" capacitance alone were present, voltage distribution throughout the winding would be substantially uniform and linear, as it would be also if inductance alone were present. Inasmuch as series and parallel distributed capacitance are inherent characteristics, the voltage distribution of impulse voltages applied to such highly inductive windings is an extremely important design consideration.
The two principal winding configurations used in power transformers of high voltage and current rating are the "layer" type formed as a cylindrical helix or groups of concentric helices and the radial spiral "disc" type. In a continuous disc-type winding, each of a plurality of annular coils is wound as a radial spiral, the coils (i.e., radial spirals) being disposed in axial juxtaposition on a linear core and connected electrically in a series circuit relation.
It is also well known that a layer-type winding has a more linear transient voltage distribution than does a continuous disc-type winding, because the series capacitance of a layer winding is large relative to its parallel capacitance. However, for some high-voltage applications the disc-type winding is used in order to avoid a high voltage gradient (and consequent heavy insulation) between helical layers at normal operating voltages. Thus, medium power high-voltage transformers often have low-voltage windings of the layer type and high-voltage windings of the disc type. In such transformers the low-voltage winding is commonly located immediately adjacent the core and is surrounded by the higher voltage disc winding. Relative to the high-voltage winding, the entire low-voltage winding is approximately at ground potential, and the radial space between them, called the "main gap," is an essential design parameter. The radial dimension of the main gap is determined primarily by two considerations. One is the maximum permissible voltage stress accross the main gap at the low, power-circuit frequency and the other is the voltage stress arising from high-frequency transient voltages. In practice the latter consideration often controls the size of the main gap in disc-type transformers. In disc windings with adjacent winding coils connected in a series circuit relation (i.e., a continuous disc winding) the non-linearity of coil-to-coil impulse voltage stress usually requires that the first several turns at the high-voltage end be provided with extra insulation. For reasons of economy and size it is desirable to reduce the size of the main gap and to reduce the amount of insulation between disc coils and between coil turns. All of these results may be accomplished if the normally steep exponential impulse voltage distribution, which particularly characterizes the continuous disc winding, can be favorably modified and brought closer to an ideal uniform linear distribution.
It is known that the transient voltage distribution between axially juxtaposed coils or groups of coils in a disc-type winding may be improved by various expedients which increase series capacitance relative to parallel capacitance. One such expedient is to place one or more shielding conductors between coil turns of the disc coils of a winding, as illustrated in U.S. Pat. No. 2,905,911 to KURITA. It is also known that these shield conductors or electrostatic shields become less effective as the distance from the high potential end of a winding to the electrostatic shield increases. Placing electrostatic shields, of the just mentioned type along the entire length of a disc-wound winding is considered poor design practice because of cost and size considerations and such designs are usually avoided. While it is true that more electrostatic shields will, in fact, improve the transient response of a disc-wound winding there is a region in such a winding, which is some calculable distance from a high potential end of same, where a point of diminishing returns is reached. Providing additional electrostatic shields beyond this region of the winding will result in a degree of transient response improvement that is not justified by the penalty that must be paid to obtain this improved response in terms of increased winding size and cost. Normal design practice is to discontinue electrostatic shields beyond this calculable distance. However, discontinuing such electrostatic shields other than at the end of a disc-wound winding creates problems that would not be present if electrostatic shields were continued throughout its entire length.
Abrupt changes in series capacitance occur when going from that portion of a disc-wound winding having electrostatic shields, hereinafter designated the compensated portion, to that portion of the same winding that does not have electrostatic shields, hereinafter designated the uncompensated portion. This sudden change in series capacitance results in unsatisfactory transient voltage build-up at the beginning of the low series capacitance or uncompensated portion of the winding. If possible, sudden changes in series capacitance of such disc-wound windings should be minimized to, in turn, minimize said transient voltage build-up.